Post parallel ledger close cleanup

[marta-lokhova]

To do after #4543 is landed (opening a separate issue to reduce the scope of this PR as per discussion with @graydon):

• Audit gRandomEngine multi-threaded access
• Rename CatchupManager -> ApplyExternalizedLedgersManager
• Remove app from BucketManager/FutureBucket or at a minimum enforce no access to LedgerManager from BucketManager (to guarantee no deadlocks)
• Enforce that only one thread can reference an active LedgerTxn at all times
• Remove LM from valueExternalized flow in Herder. Herder should go directly to LedgerApplyManager, since it is the module that handles applying now.

This will also resolve #4318